This invention generally relates to the field of ink jet printing. In particular, this application relates to ink compositions for ink jet printing onto plasticized vinyl substrates and to the resultant printed articles.
Ink jet printing is a well known technique by which printing is effected without contact between the printing device and the substrate on which the printed characters are deposited. Describing briefly one technique for doing ink jet printing, a stream of droplets of ink are projected toward a surface and the trajectories of the droplets are controlled electronically so that the droplets are caused to form the desired printed image on a relatively movable substrate. Another technique of ink jet printing is to direct droplets on demand from a set of orifices at a relatively moving substrate. Such non-contact ink jet printing techniques are particularly well suited for application of characters onto irregularly shaped surfaces.
In general, ink jet printing techniques impose rigid requirements on the ink compositions. To be suitable for use as a jet ink, the compositions must meet some or all of the requirements of viscosity, resistivity, solubility, compatibility of components and wetability of the substrate. The ink must be quick drying and smear resistant and capable of passing through the ink jet nozzle without clogging. The ink should also permit rapid cleanup of the machine components with minimum effort.
Many ink jet printing compositions contain a carrier and a dye which is soluble or dispersible in that carrier, as well as evaporation retardants to control evaporative losses from the system, resistivity control agents to adjust the electrical resistivity of the ink and other conventional components. For many applications, the jet ink composition also contains a binder component such as a vinyl acetate copolymer to improve adhesion between the printed image and the substrate on which the printing is effected. A variety of such resin binders have been employed, as described in U.S. Pat. Nos. 4,210,566, 4,260,531, 4,567,213 and 4,070,322. In practice, ink jet printing is applied to several different types of substrates, such as paper, glass, metal, and plastic.
Unfortunately, it has been found that although prior jet ink compositions containing resin binders have been generally suitable for jet printing onto most substrates, a problem has existed when the substrate is of the vinyl polymer type, containing one or more plasticizers. In such an instance, the quality of printed characters made with the prior art ink compositions has been found to deteriorate with the passage of time. This deterioration problem is of particular concern, as one of the important commercial applications for ink jet printing is the coding of insulated wires. Ink jet printing is particularly well suited to such an application because the irregularities of the surface of the insulating coating on the wires make it difficult, if not impossible, to employ conventional printing techniques. However, the insulating coating on the wires commonly is of the plasticized vinyl polymer type. Because of the aforementioned deterioration of the printed images made with prior art inks, such compositions when printed onto the insulated wires have produced unsatisfactory results. Apparently, an interaction exists between the plasticizer for the vinyl coating and the printed image which causes the printed image to fade with time to the point that it eventually becomes illegible.
Thus, a need has existed in the art for an ink composition which is suitable for ink jet printing onto plasticized vinyl substrates, such as those used to insulate wires, to form printed images that are stable.
It is, therefore, an object of the present invention to provide an ink composition which overcomes the deterioration problem associated with the use of prior jet ink formulations when applied to plasticized vinyl substrates. Specifically, a principal object of the present invention is to provide an ink composition suitable for ink jet printing onto vinyl polymer substrates, which is able to produce printed images which resist fading caused by interaction with the plasticizer in the vinyl substrate. The foregoing and other objects of the invention will be apparent from the following description of the present invention.